Fingerprint sensing and matching is a reliable and widely used technique for personal identification or verification. In particular, a common approach to fingerprint identification involves scanning a sample fingerprint or an image thereof and storing the image and/or unique characteristics of the fingerprint image. The characteristics of a sample fingerprint may be compared to information for reference fingerprints already in a database to determine proper identification of a person, such as for verification purposes.
In recent years it has been practical and economical to build high-quality electronic fingerprint sensing devices using radio-frequency (RF) electric fields to develop an electronic representation of the fingerprint pattern. One such RF fingerprint sensing device is disclosed in U.S. Pat. No. 5,940,526 to Setlak et al. and assigned to the assignee of the present invention. The patent discloses an integrated circuit fingerprint sensor including an array of RF sensing electrodes to provide an accurate image of the fingerprint friction ridges and valleys. More particularly, the RF sensing permits imaging of live tissue just below the surface of the skin to reduce spoofing, for example. Multiple biometric characteristics sensed from the user's finger may also be used as disclosed in Published U.S. Patent Application No. 2005/0089203 also to Setlak et al., and assigned to the assignee of the present invention. The entire contents of the Setlak et al. patent and published application are incorporated herein by reference.
One important feature of a fingerprint sensor is the ability to detect a finger from a low power state. Typically a user may require the sensor to be in a low power state while no finger is positioned on the sensor. This relaxes the burden on the power supply that may be especially important for portable handheld devices, such as cellular phones, PDA's, etc.
U.S. Pat. No. 6,844,660 to Scott discloses a finger sensor comprising an array of piezoelectric pixels, and that wake-ups the circuitry upon detecting a finger pressing upon the array. AuthenTec Inc. of Melbourne, Fla., the assignee of the present invention, has used two finger detection approaches: a first using a hardware delay, and a second based upon image detection. The hardware delay approach includes a calibrated internal resistor-capacitor (RC) delay path as a comparison to a fixed internal resistance in conjunction with the finger capacitance. The sensor wakes up once the finger comes into contact with the electrode ring based upon the internal finger detect comparator changing state. This approach maybe sufficient for static placement sensors, but has some shortcomings for smaller swipe or slide finger sensors where perspiration can accumulate and remain on the sensor. This may cause image data to continue to run and which may eventually end the communication interface. This hardware approach may be unsatisfactory because perspiration accumulation may cause enough delay, or capacitance, to erroneously trip the finger detect comparator.
In the image-based finger detect approach, image data is collected and can be compared after an analog-to-digital (A/D) conversion. This approach addresses the undesired effects of perspiration accumulation, but at the expense of greater power consumption. Further aspects of exemplary finger sensors and wake-up or finger detect circuitry and operation are disclosed in U.S. Pat. Nos. 5,940,526 and 6,628,812, both assigned to the assignee of the present invention and the entire contents of which are incorporated herein by reference.